Photodamage mitigation compounds and systems

ABSTRACT

Compositions, devices, systems and methods for reducing and/or preventing photo-induced damage of one or more reactants in an illuminated analytical reaction by addition of one or more photoprotective compounds to the reaction mixture and allowing the reaction to proceed for a period that is less than a photo-induced damage threshold period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/208,151 filed Aug. 11, 2011, which claims priority from ProvisionalU.S. Patent Application No. 61/401,471, filed Aug. 12, 2010; andProvisional U.S. Patent Application No. 61/466,734, filed Mar. 23, 2011,the full disclosures of which are incorporated herein by reference intheir entireties for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The use of optically detectable labeling groups, and particularly thosegroups having high quantum yields, e.g., fluorescent, phosphorescent,luminescent or chemiluminescent groups, is ubiquitous throughout thefields of analytical chemistry, biochemistry, and biology. Inparticular, by providing a highly visible signal associated with a givenreaction, one can better monitor that reaction as well as any potentialeffectors of that reaction. Such analyses are the basic tools of lifescience research in genomics, diagnostics, pharmaceutical research, andrelated fields.

Such analyses have generally been performed under conditions where theamounts of reactants are present far in excess of what is required forthe reaction in question. The result of this excess is to provide ampledetectability, as well as to compensate for any damage caused by thedetection system and allow for signal detection with minimal impact onthe reactants. For example, analyses based on fluorescent labelinggroups generally require the use of an excitation radiation sourcedirected at the reaction mixture to excite the fluorescent labelinggroup, which is then separately detectable. However, one drawback to theuse of optically detectable labeling groups is that prolonged exposureof chemical and biochemical reactants to such light sources, alone, orwhen in the presence of other components, e.g., the fluorescent groups,can damage such reactants, e.g., proteins, enzymes, and the like. Thetraditional solution to this drawback is to have the reactants presentso far in excess that the number of undamaged reactant molecules faroutnumbers the damaged reactant molecules, thus minimizing or negatingthe effects of the photo-induced damage.

A variety of analytical techniques currently being explored deviate fromthe traditional techniques. In particular, many reactions are based onincreasingly smaller amounts of reagents, e.g., in microfluidic ornanofluidic reaction vessels or channels, or in “single molecule”analyses. Such low reactant volumes are increasingly important in manyhigh throughput applications, such as microarrays.

The use of smaller reactant volumes offers challenges to the use ofoptical detection systems. When smaller reactant volumes are used,damage to reactants, such as from exposure to light sources forfluorescent detection, can become problematic and have a dramatic impacton the operation of a given analysis. This can be particularlydetrimental, for example, in real time analysis of reactions thatinclude fluorescent reagents that can expose multiple different reactioncomponents to optical energy. In addition, smaller reactant volumes canlead to limitations in the amount of signal generated upon applicationof optical energy.

As such, methods and compositions that result in increased effectiveconcentrations of reactants and detection molecules in smaller reactantvolumes, thereby increasing signal in a smaller volume, would provideuseful improvements to the methods and compositions currently available.For example, methods and compositions that prevent or mitigate to someextent photo-induced damage in a reaction of interest would beparticularly useful.

BRIEF SUMMARY OF THE INVENTION

The present invention is generally directed to compounds, compositions,methods, devices and systems for preventing, reducing, or limiting theeffects of photo-induced damage during illuminated reactions,particularly reactions that employ fluorescent or fluorogenic reactants.The term “photo-induced damage” refers generally to any direct orindirect impact of illumination on one or more reagents in a reactionresulting in a negative impact upon that reaction. The term “illuminatedreactions” as used herein refers to reactions which are exposed to anoptical energy source. Typically, such illumination is provided in orderto observe the generation and/or consumption of reactants or productsthat possess a particular optical characteristic indicative of theirpresence, such as an alteration (in intensity, wavelength, etc.) in theabsorbance spectrum and/or emission spectrum of the reaction mixture orits components.

In a first aspect, the invention provides reaction mixtures that includea fluorescent or fluorogenic molecule, and a photoprotective agent, thephotoprotective agent including a reducing unit covalently bound to anoxidizing unit. In certain preferred embodiments, the photoprotectiveagent further comprises a water solubilizing unit. Optionally includedin the mixtures is a nucleoside polyphosphate (or analog thereof) and/oran enzyme, e.g., a polymerase or ligase enzyme. The mixtures can furtherinclude a template nucleic acid molecule. At least one component of thereaction mixture can be confined within a zero mode waveguide.Preferably, at least one component of the reaction mixture is linked toa fluorescent or fluorogenic molecule. In certain specific embodiments,one component of the reaction mixture is a fluorescently labeled enzyme,nucleotide polyphosphate, polynucleotide, tRNA, amino acid, or analogthereof.

Reaction mixtures of the invention preferably include a photoprotectiveagent that reduces an amount of photo-induced damage to one or morereaction components that would otherwise occur in the absence of thephotoprotective agent. In preferred embodiments, the photoprotectiveagent is a single molecule reducing and oxidizing system, a nitrobenzenederivative (e.g., nitrobenzoic acid or a derivative thereof), and thelike. The reducing unit can include, e.g., one or more of a thiol group,a disulfide group, ascorbic acid, an ascorbic acid derivative,dialkylaniline, a dialkylaniline derivative, anthracene, an anthracenederivative, an aliphatic amine, and/or an aromatic amine. The oxidizingunit can include, e.g., one or more of a nitro group, a quinone orderivative thereof, a hydroquinone or derivative thereof,methylviologen, a methylviologen derivative, a nitrobenzene derivative,nitrobenzoic acid, or a nitrobenzoic acid derivative. Alternatively, insome preferred embodiments the photoprotective agent does not comprise aquinone, hydroquinone, or a derivative thereof. Optionally, the reactionmixtures include a photoprotective agent that includes a compound of aformula selected from the group consisting of:

where a carboxyl group may also be a carboxylate salt thereof, e.g.,potassium carboxylate; and where a carboxylate salt may be a carboxylgroup or a different carboxylate salt (a carboxylate anion bound to acationic unit different from the one (e.g., K⁺) illustrated).

In a related aspect, the invention provides reaction mixtures thatinclude a first reactant, a second reactant comprising a fluorescent orfluorogenic label, and a photoprotective agent, the photoprotectiveagent including a reducing unit covalently bound to an oxidizing unit.In certain preferred embodiments, the photoprotective agent furthercomprises a water solubilizing unit. Here, interaction of the first andsecond reactants under excitation illumination causes photo-induceddamage to the first reactant in the absence of the photoprotectiveagent, which reduces an amount of photo-induced damage to at least thefirst reactant. Optionally, the first reactant is a polymerase, ligase,nuclease, or a ribosome. The reaction mixtures optionally include aphotoprotective agent that is a single molecule reducing and oxidizingsystem, e.g., a nitrobenzene derivative (e.g., nitrobenzoic acid or aderivative thereof), and the like. The reducing unit can include, e.g.,one or more of a thiol group, a disulfide group, ascorbic acid, anascorbic acid derivative, dialkylaniline, a dialkylaniline derivative,anthracene, an anthracene derivative, an aliphatic amine, and/or anaromatic amine. The oxidizing unit can include, e.g., one or more of anitro group, a quinone, a quinone derivative, methylviologen, amethylviologen derivative, a nitrobenzene derivative, nitrobenzoic acid,or a nitrobenzoic acid derivative. Alternatively, in some preferredembodiments the photoprotective agent does not comprise a quinone,hydroquinone, or a derivative thereof. Optionally, reaction mixtures inthis aspect of the invention include a photoprotective agent thatincludes a compound of a formula selected from the group consisting of:

where a carboxyl group may also be a carboxylate salt thereof, e.g.,potassium carboxylate; and where a carboxylate salt may be a carboxylgroup or a different carboxylate salt (a carboxylate anion bound to acationic unit different from the one (e.g., K⁺) illustrated).

Reaction mixtures that include a polymerase, nuclease, or ligase enzymecan further include a template nucleic acid molecule and/or a nucleosidepolyphosphate or analog thereof. Reaction mixtures that include aribosome can further include mRNA, tRNA, amino acids, and otherreactants typically included in a translation reaction. Optionally, atleast one component of the reaction mixture is confined within a zeromode waveguide.

Methods for protecting an enzyme from photo-induced damage in anilluminated reaction are also provided by the present invention. Themethods include providing a reaction mixture that includes the enzymeand a fluorescent or fluorogenic substrate for the enzyme. Interactionof the enzyme and the fluorescent or fluorogenic substrate underexcitation illumination results in altered activity of the enzyme. Suchinteraction may be a covalent or noncovalent interaction, bindinginteraction, a transient interaction, a catalytic interaction, and thelike. In accordance with the methods of the invention, the reactionmixtures can further include a template nucleic acid molecule and/or anucleoside polyphosphate or analog thereof. Optionally, at least onecomponent of the reaction mixture is confined within a zero modewaveguide.

The methods further comprise adding a photoprotective agent to thereaction mixture, the photoprotective agent including a reducing unitcovalently bound to an oxidizing unit (e.g., a single molecule reducingand oxidizing system). In certain preferred embodiments, thephotoprotective agent further comprises a water solubilizing unit. Inpreferred embodiments, the photoprotective agent is a nitrobenzenederivative (e.g., nitrobenzoic acid or a derivative thereof). Thereducing unit can include, e.g., one or more of a thiol group, adisulfide group, ascorbic acid, an ascorbic acid derivative,dialkylaniline, a dialkylaniline derivative, anthracene, an anthracenederivative, an aliphatic amine, and/or an aromatic amine. The oxidizingunit can include, e.g., one or more of a nitro group, a quinone orderivative thereof, a hydroquinone or derivative thereof,methylviologen, a methylviologen derivative, a nitrobenzene derivative,nitrobenzoic acid, or a nitrobenzoic acid derivative. Alternatively, insome preferred embodiments the photoprotective agent does not comprise aquinone, hydroquinone, or a derivative thereof. Optionally, the reactionmixtures include a photoprotective agent that includes a compound of aformula selected from the group consisting of:

where a carboxyl group may also be a carboxylate salt thereof, e.g.,potassium carboxylate; and where a carboxylate salt may be a carboxylgroup or a different carboxylate salt (a carboxylate anion bound to acationic unit different from the one (e.g., K⁺) illustrated).

The methods further include illuminating the reaction mixture withexcitation illumination. The photoprotective agent reduces an amount ofphoto-induced damage to the enzyme resulting from interaction of theenzyme with the fluorescent or fluorogenic substrate under theexcitation illumination to an amount that is less than that which wouldoccur in the absence of the photoprotective agent. Such interaction maybe a covalent or noncovalent interaction, binding interaction, atransient interaction, a catalytic interaction, and the like.Optionally, the reaction mixture is illuminated for a period of timethat is less than a photo-induced damage threshold period, wherein thephoto-induced damage threshold period is lengthened in the presence ofthe photoprotective agent.

Optionally, methods of the invention further include the step ofmonitoring a reaction between the enzyme and the fluorescent orfluorogenic substrate while illuminating the reaction mixture. In somepreferred embodiments, the illuminated reaction is a base extensionreaction and the enzyme is optionally a polymerase. Such a reactiontypically includes at least one polynucleotide (e.g., template) and aplurality of nucleotide polyphosphates. In other preferred embodiments,the illuminated reaction is a translation reaction during which apolypeptide is synthesized. Such a reaction typically includes aribosome, and mRNA, and a plurality of amino acid-charged tRNAs.Preferably, one or more component of the reaction mixture are confinedupon a substrate, e.g., within a zero mode waveguide.

Also provided by the present invention are compounds having a formulaselected from:

where a carboxyl group may also be a carboxylate salt thereof, e.g.,potassium carboxylate; and where a carboxylate salt may be a carboxylgroup or a different carboxylate salt (a carboxylate anion bound to acationic unit different from the one (e.g., K⁺) illustrated).

In another aspect, the present invention provides devices that include asubstrate having an observation region (e.g., an observation regionwithin a zero mode waveguide), a first reactant (e.g., an enzyme, e.g.,a polymerase, ligase, nuclease, or a ribosome) immobilized within theobservation region, a second reactant disposed within the observationregion, where interaction between the first and second reactants underexcitation illumination causes photo-induced damage to the firstreactant. Where the first reactant is an enzyme, the second reactant ispreferably a fluorescent or fluorogenic substrate for the enzyme. Inpreferred embodiments, at least one or more components of the reactionmixture comprising the first and second reactants is confined upon asubstrate, e.g., within a zero mode waveguide. The devices furtherinclude a photoprotective agent disposed within the observation region,the photoprotective agent including a reducing unit covalently bound toan oxidizing unit. In certain preferred embodiments, the photoprotectiveagent further comprises a water solubilizing unit. Certain preferredphotoprotective agents are nitrobenzene derivatives. In someembodiments, the photoprotective agent does not comprise a quinone,hydroquinone, or a derivative thereof. Devices of the inventionoptionally include a photoprotective agent that includes at least onecompound of a formula selected from the group consisting of:

where a carboxyl group may also be a carboxylate salt thereof, e.g.,potassium carboxylate; and where a carboxylate salt may be a carboxylgroup or a different carboxylate salt (a carboxylate anion bound to acationic unit different from the one (e.g., K⁺) illustrated).

Also provided by the present invention are systems for analyzing anilluminated reaction (e.g., a sequencing reaction, a base extensionreaction, etc.) that is susceptible to photo-induced damage whenilluminated for a period longer than a photo-induced damage thresholdperiod. Such systems include a mounting stage configured to receive thesubstrate, an optical train positioned to be in optical communicationwith at least a portion of the substrate to illuminate the portion ofthe substrate and detect signals emanating therefrom, a translationsystem operably coupled to the mounting stage or the optical train formoving one of the optical train and the substrate relative to the other,and a substrate having one or more reagents for the reaction disposedthereon, where at least one of the reagents is a photoprotective agentcomprising a reducing unit covalently bound to an oxidizing unit. Incertain preferred embodiments, the photoprotective agent furthercomprises a water solubilizing unit. In certain embodiments, thephotoprotective agent comprises a nitrobenzene derivative, andadditionally or alternatively, does not comprise a quinone,hydroquinone, or a derivative thereof. Optionally, the substrateincludes at least one zero mode waveguide. The one or morephotoprotective agents optionally include a compound of the formulaselected from:

where a carboxyl group may also be a carboxylate salt thereof, e.g.,potassium carboxylate; and where a carboxylate salt may be a carboxylgroup or a different carboxylate salt (a carboxylate anion bound to acationic unit different from the one (e.g., K⁺) illustrated).

In a further aspect, the present invention provides methods forpreparing an illuminated reaction mixture (e.g., a sequencing orbase-extension reaction mixture) that comprises fluorescent offluorogenic compounds. The methods include adding a photoprotectiveagent to the reaction mixture, the photoprotective agent including areducing unit covalently bound to an oxidizing unit, where thephotoprotective agent reduces an amount of photo-induced damage to atleast one component of the illuminated reaction mixture that wouldotherwise occur in the absence of the photoprotective agent. In certainpreferred embodiments, the photoprotective agent further comprises awater solubilizing unit. The reducing unit optionally includes one ormore of a thiol group, a disulfide group, ascorbic acid, an ascorbicacid derivative, dialkylaniline, a dialkylaniline derivative,anthracene, an anthracene derivative, an aliphatic amine, and/or anaromatic amine. Optionally, the oxidizing unit includes one or more of anitro group, a quinone, a quinone derivative, methylviologen, amethylviologen derivative, a nitrobenzene derivative, nitrobenzoic acid,or a nitrobenzoic acid derivative. In some preferred embodiments, thephotoprotective agent does not comprise a quinone, hydroquinone, or aderivative thereof. The photoprotective agent is preferably atriplet-state quencher, a nitrobenzene derivative (e.g., nitrobenzoicacid or a derivative thereof), or a photoprotective agent that includesa compound of a formula selected from the group consisting of:

where a carboxyl group may also be a carboxylate salt thereof, e.g.,potassium carboxylate; and where a carboxylate salt may be a carboxylgroup or a different carboxylate salt (a carboxylate anion bound to acationic unit different from the one (e.g., K⁺) illustrated).Optionally, the methods further comprise confining at least onecomponent of the reaction mixture on a substrate, e.g., a substrate thatincludes one or more zero mode waveguides.

In another aspect, the present invention provides methods for increasingthe accuracy of an illuminated sequencing reaction. The methods includeproviding a reaction mixture that includes a polymerase, a templatenucleic acid, one or more fluorescent or fluorogenic nucleotides ornucleotide analogs, and a photoprotective agent that comprises areducing unit covalently bound to an oxidizing unit, and optionallyfurther comprising a water solubilizing unit. The reaction mixture isexposed to excitation illumination and emission signals are detectedfrom the reaction mixture, e.g, during monitoring of the reactionmixture during the exposure to excitation illumination. The presence ofthe photoprotective agent enhances the accurate detection of thefluorescent or fluorogenic nucleotides, thereby increasing the accuracyof the resulting sequencing reaction data. For example, thephotoprotective agent can decrease an amount of blinking and/orphotobleaching of a dye within the fluorescent or fluorogenicnucleotides or nucleotide analogs. Such blinking and/or photobleachingcan result from exposure of the dye to the excitation illumination.Addition of the photoprotective agent reduces the amount of blinkingand/or photobleaching that would otherwise occur in the absence of thephotoprotective agent. In certain preferred embodiments, the illuminatedsequencing reaction is a base extension reaction, e.g., atemplate-directed nascent strand extension reaction. Optionally, atleast one component of the reaction mixture is confined within a zeromode waveguide, e.g., the polymerase or the template nucleic acid.

The photoprotective agent is preferably a single molecule ROXS compoundas provided elsewhere herein. For example, the photoprotective agent cancomprise a nitrobenzene derivative, and/or can optionally not comprise aquinone, hydroquinone, or a derivative thereof. A photoprotective agentcan include a compound of a formula selected from the group consistingof:

where a carboxyl group may also be a carboxylate salt thereof, e.g.,potassium carboxylate; and where a carboxylate salt may be a carboxylgroup or a different carboxylate salt (a carboxylate anion bound to acationic unit different from the one (e.g., K⁺) illustrated).

The present invention also provides kits that incorporate photo-induceddamage mitigating agents, or admixtures thereof, optionally withadditional useful reagents. Such kits typically include a photo-induceddamage mitigating agent of the invention packaged in a fashion to enableuse of the agent with any of a variety of analytical reaction componentssusceptible to photo-induced damage that participate in a reaction withone or more fluorescent or fluorogenic reaction components. For example,a photo-induced damage mitigating agent of the invention can be packagedwith any of a variety of enzymes that participate in a reaction with oneor more fluorescent or fluorogenic substrates. Alternatively, aphoto-induced damage mitigating agent of the invention can be packagedwith any of a variety of antibodies that participate in a reaction withone or more fluorescent or fluorogenic antigens, or vice versa. In stillother embodiments, a photo-induced damage mitigating agent of theinvention can be packaged with any of a variety of protein receptorsthat participate in a reaction with one or more fluorescent orfluorogenic ligands. It will be clear that the methods, compositions,and systems described herein are useful with a multitude of other typesof analytical reactions, including but not limited to hybridizationassays, binding assays (e.g., antibody assays), nucleic acid sequencingassays, protein sequencing assays, polymerization assays, ligationreactions, catalytic reactions, etc. Depending upon the desiredapplication, the kits of the invention optionally include, e.g., buffersolutions and/or salt solutions, divalent metal ions, i.e., Mg⁺⁺, Mn⁺⁺,Ca⁺⁺, Zn⁺⁺ and/or Fe⁺⁺, enzyme cofactors, substrates, standardsolutions, e.g., dye standards for detector calibration, etc. Kits canoptionally include reagents and instructions for preparing photo-induceddamage mitigating agent admixtures. Such kits also typically includeinstructions for use of the compounds and other reagents in accordancewith the desired application methods, e.g., nucleic acid sequencing andthe like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a proposed mechanism ofphoto-induced damage to DNA polymerase in template-dependent synthesisusing fluorescent nucleotide analogs while under excitationillumination.

FIG. 2 provides certain exemplary embodiments of the photoprotectiveagents provided herein.

FIG. 3 provides additional photoprotective agents of the invention,e.g., hydroxyjulolideine derivatives.

FIG. 4 is a Jablonski diagram illustrating excited vibrational states.

FIG. 5 shows accuracy distributions for sequencing reactions in thepresence of a two-component ROXS (panel A), a single molecule ROXScompound (panel B), and in the absence of both a two-component ROXS anda single molecule ROXS compound (panel C).

FIG. 6 shows accuracy distributions for these reactions in the presenceof a two different single molecule ROXS compounds (panels A and B) andin the absence of both (panel C), as well as the chemical structures ofthe compounds.

FIG. 7 shows accuracy distributions for these reactions in the presenceof a two different two-component ROXS (panels A and B), and twodifferent single molecule ROXS compounds (panels C and D).

FIG. 8 shows accuracy distributions for these reactions in the presenceof a two-component ROXS (panels A and E), and six different singlemolecule ROXS compounds (panels B, C, D, F, G, and H), as well as thechemical structures of the compounds.

FIG. 9 shows accuracy distributions for these reactions in the presenceof four different single molecule ROXS compounds (panels A-D), and thechemical structures of the compounds are provided to the left of thedistributions.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. All publications mentionedherein are incorporated herein by reference for the purpose ofdescribing and disclosing devices, formulations and methodologies whichare described in the publication and which might be used in connectionwith the presently described invention.

Note that as used herein and in the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a polymerase”refers to one agent or mixtures of such agents, and reference to “themethod” includes reference to equivalent steps and methods known tothose skilled in the art, and so forth. Where a range of values isprovided, it is understood that each intervening value, between theupper and lower limit of that range and any other stated or interveningvalue in that stated range is encompassed within the invention. Theupper and lower limits of these smaller ranges may independently beincluded in the smaller ranges, and are also encompassed within theinvention, subject to any specifically excluded limit in the statedrange. Where the stated range includes one or both of the limits, rangesexcluding either both of those included limits are also included in theinvention.

In the following description, numerous specific details are set forth toprovide a more thorough understanding of the present invention. However,it will be apparent to one of skill in the art that the presentinvention may be practiced without one or more of these specificdetails. In other instances, well-known features and procedures wellknown to those skilled in the art have not been described in order toavoid obscuring the invention.

I. General

The present invention is generally directed to compounds, compositions,methods, devices and systems for preventing, reducing, or limiting theeffects of photo-induced damage during illuminated reactions,particularly reactions that employ fluorescent or fluorogenic reactants.Fluorescent or fluorogenic reactants generally include reactioncomponents linked to a fluorescent or fluorogenic molecule or “label.”Such reaction components include without limitation enzymes, enzymesubstrates, cofactors, reactive proteins, binding partners, and othertypes of molecules desired to be detected during an analytical reaction.Further, in some embodiments, a fluorescent or fluorogenic molecule canbe linked to a reaction site rather than, or in addition to, a reactioncomponent. The present invention provides methods and compositions forprotecting reaction components (e.g., enzymes, enzyme substrates,ligands, or fluorescent dyes) from photo-induced damage that can alterthe activity of the reaction components. The term “photo-induced damage”refers generally to any direct or indirect impact of illumination on oneor more reagents or other components in a reaction resulting in anegative impact upon that reaction. The term “illuminated reactions” asused herein refers to reactions which are exposed to an optical energysource. Typically, such illumination is provided in order to observe thepresence (e.g., generation, binding, and/or consumption) of reactants orproducts that possess a particular optical characteristic indicative oftheir presence, such as a shift in the absorbance spectrum and/oremission spectrum of the reaction mixture or its components or a changein intensity of fluorescence.

For example, the fluorescence detected in a fluorescence-based opticalassay is the result of a three-stage process that occurs in thefluorophores or fluorescent dyes present in a reaction mixture. Thefirst stage is excitation in which a photon with quantized energy froman external light source having a specific wavelength (e.g., from alaser) is supplied and absorbed by a fluorophore creating an excitedelectronic singlet state (S₁′). The second stage is the excited-statelifetime in which the excited fluorophore undergoes several differentchanges to relax its energy to the lowest singlet state (S₁). From theS₁ state several possible mechanisms can occur in the third stage,fluorescence, in which a photon of energy (S₁-S₀) is emitted returningthe fluorophore to its ground state. Many thousands of these three-stageprocesses of excitation and emission typically occur to produce a signaldetectable by standard optical sensors.

One of the many pathways that dissipate the energy of the excitedelectronic singlet state (S₁) is the intersystem crossing (ISC), whichinvolves a change in spin multiplicity that transits the system from S₁to the excited triplet state (T₁). In many fluorescent dye molecules,the formation of the much longer lifetime triplet-state species competeswith fluorescence emission and greatly reduces the brightness of thefluorescence emission. In addition, fluorescent dyes exhibit a highdegree of chemical reactivity in this state that often results inphotobleaching and the production of damaging free radicals and reactiveintermediates, e.g., radical ions, carbenes, carbocations, carbanions,etc. Further, there is also evidence to suggest that even a fluorophorein the S₁ state can react with reaction components, e.g., negativelyimpacting an analytical reaction by mediating the production ofnon-reactive intermediates.

In general terms, the invention is directed to the performance ofilluminated reaction analyses, where such analyses are illuminated foran amount of time that permits the effective performance of theanalysis. In some embodiments, one or more photo-induced damagemitigating agents (e.g., reducing agents, oxidizing agents,triplet-state quenchers, free radical quenchers, oxygen scavengers,and/or a combination thereof) may be included in an illuminatedreaction. Certain examples of photo-induced damage mitigating agents areprovided in U.S. Patent Publication Nos. 2007/0128133, 2007/0161017, and2010/0136592, all of which are incorporated herein by reference in theirentireties for all purposes. In certain embodiments, compounds of theinvention function as triplet-state quenchers and/or free radicalquenchers, e.g., to prevent, slow, or remove the accumulation ofdamaging excited triplet-state forms of one or more reaction components.For example, in specific embodiments, using photo-induced damagemitigating agents of the invention slows the accumulation of the excitedtriplet state of the fluorophore by, e.g, reducing T1 lifetime andrestoring the fluorophore to its ground state (S0) (thereby facilitatingthe availability of the fluorophore to absorb another photon andfluoresce again), greatly improving the photophysical properties of thedye. This reduction in triplet state lifetime also reduces thelikelihood that other reaction components will undergo photo-induceddamage caused by interaction with a triplet-state dye, therebyessentially protecting the other reaction components and potentiallyextending the time during which the reaction can generate useful data.

Photoprotective compounds can be added to the reaction mixture, e.g., toform a triplet state quenching (TSQ) system, e.g., with multiple TSQreagents added to address the triplet states for additional dyes orfluorophores used in the reaction at issue. A reducing and oxidizingsystem (ROXS) can be effective in minimizing photobleaching and blinkingof fluorescent dyes. (Agnew. Chem. Int. Ed, 2008, 47, 5465-5469,incorporated herein by reference in its entirety for all purposes.) Suchphotobleaching and blinking can adversely affect the detectability of anemission from a day, so the presence of an ROXS can facilitate detectionat least by reducing these photochemical phenomena. Typically, an ROXSis a multicomponent system, e.g., comprising a pair of TSQ reagents thatincludes a reducing agent and an oxidizing agent, which can work inconcert to speed the relaxation of a dye from its triplet state to itsground state. For example, one reagent reduces the dye triplet stateinto a radical anion which can then be oxidized by the second reagentback to ground state. Alternatively, the second reagent oxidizes the dyetriplet state into a radical cation which can then be reduced by thefirst reagent into its ground state. An example of such a system is amixture of methyl-viologen and ascorbic acid. Another ROXS that can beused is one including nitrobenzoic acid or salts thereof andmercaptoethylamine, e.g., 2-mercaptoetylamineHCL and sodium 2-, 3-and/or 4-nitrobenzoate. Other systems include mixtures of nitrobenzoicacid or salts thereof (e.g., potassium 2-nitrobenzoic acid) and otherreducing agents, such as DTT (dithiothreitol) or DMAPA(dimethylaminopropylamine). Adding these reagents to an illuminatedreaction can be used to mitigate photo-induced damage to otherreactants, e.g., an enzyme, and to improve other reaction metrics, aswell. For example, a reduction in photobleaching and/or blinking canimprove the detectability of fluorescent or fluorogenic reactioncomponents, which can increase the accuracy with which their presence ismonitored. However, a drawback to this type of system, e.g., when usedin single molecule systems is that relatively high concentrations ofeach TSQ reagent are needed because it requires two differentbimolecular reactions, one with each TSQ reagent, to complete thetriplet state quenching. As such, the rate of completion of the tworeactions is diffusion-limited and dependent upon multiple independentmolecular collision events. Further, certain analytical reactions areadversely affected by the high concentrations of agents required in amulticomponent ROXS. For example, in a single molecule enzyme reaction,the high concentrations of the ROXS reagents can lead to reduced enzyme(e.g., polymerase, ribosome, nuclease, etc.) activity, e.g., resultingin suboptimal product generation, e.g., reduced read length in a singlemolecule sequencing reaction.

In certain preferred embodiments, the invention provides methods andcompositions for nucleic acid analysis in which a photo-induced damagemitigating agent (also referred to as a photoprotective agent) that is asingle compound comprising both a reducing unit (e.g., a functionalgroup comprising a reducing agent, center or moiety) and an oxidizingunit (e.g., a functional group comprising an oxidizing agent, center ormoiety) is added to the reaction mixture. This is a preferred method ofmitigating photo-induced damage, especially in single moleculereactions, as compared to a method wherein two different compounds (onebeing a reducing agent and the other an oxidizing agent) must both beadded to the reaction mixture. When added separately, reducing andoxidizing compounds are often added in high concentrations, e.g., 10 mMeach in order to ensure that their protective effects extend to allreactant molecules in the reaction. It is believed that mitigation ofphoto-induced damage by separately added reducing and oxidizingcompounds is diffusion-limited and requires multiple molecular collisionevents. As such, relatively high concentrations are required as twodifferent compounds must both interact with the same reactant to producethe two bimolecular reactions needed to complete a reducing andoxidizing system (ROXS) quenching of the triplet state. However, in somesmall volume reactions, providing such an excess of photo-induced damagemitigating agents can potentially interfere with the ability of areaction to proceed. Having an intramolecular interaction with both areducing unit and an oxidizing unit present on the same molecule ishighly desirable so that the coupled functions of the two reactions canoccur in a single collision, e.g., to further reduce T₁ lifetime.Therefore, a lower concentration of photoprotective agent can be usedper reaction mixture, which can mitigate negative impacts of thephotoprotective agent on the reaction, e.g., due to incompatibilitiesbetween the photoprotective agents and the reaction, while stillproviding the photoprotective properties desired in the reaction, e.g.mitigation of photo-induced damage and/or reduction of blinking orphotobleaching.

In certain embodiments, a photoprotective agent, e.g., one comprisingboth a reducing unit and an oxidizing unit covalently bound in a singlecompound, can be linked to another reaction component or to a reactionsite to bring the photo-induced damage mitigating agent into closespatial proximity to the reactants susceptible to direct or indirect(e.g., by interaction with an excited dye molecule) damage by theillumination. For example, the photo-induced damage mitigating agent maybe linked to one or more of a reactant (e.g., a substrate for an enzyme(e.g., a nucleic acid, polypeptide, sugar, or monomers thereof), afluorescent or fluorogenic label (e.g., a fluorescent dye or quantumdot), an enzyme or other reactive protein or cofactor thereof (e.g., apolymerase, ligase, receptor, antibody, or nuclease)), a reaction siteat which the reaction will take place (e.g., within a well, chip, fiber,bead, optical confinement (e.g., zero mode waveguide (ZMW)), etc.), or acombination thereof (See, e.g., U.S. Patent Publication No. 20090325260,incorporated herein by reference in its entirety for all purposes.) Incertain embodiments, the invention provides methods and compositions fornucleic acid analysis in which a nucleoside polyphosphate, e.g., havingthree to eight phosphate groups, is linked to a fluorescent dye, andwherein the compound further includes, integrated into its structure(e.g., linked directly to the dye or nucleoside polyphosphate, or to astructure connecting them, such as a scaffold or linker), aphoto-induced damage mitigating agent (e.g., one comprising both areducing and an oxidizing activity), which generally refers to any agentthat can prevent and/or mitigate damages caused directly or indirectlyby illumination, for example, by triplet/radical quenching. In otherembodiments, a photo-induced damage mitigating agent (e.g., onecomprising both reducing and an oxidizing activity) is linked to anenzyme or reactive protein that interacts with a substrate or ligandcomprising a fluorescent dye. For example, where such enzyme or reactiveprotein is immobilized at a reaction site by a linker construct, thephoto-induced damage mitigating agent can be integrated into thestructure of the linker construct. Such conjugates and compositions ofthe present invention are particularly useful in small reaction volumes,because incorporating the photo-induced damage mitigating agent into oneof the reactants or linking it to a reaction site helps to maintain theprotective effects of the agent without needing to provide the agent inan excess quantity, in part by removing diffusion-limited processespresent when the photo-induced damage mitigating agents are free insolution and completion of the reaction is therefore dependent uponmultiple independent molecular collision events. Further, because theclose proximity of the photo-induced damage mitigating agents to thereactants can hasten the removal or reversal of a radical formed duringillumination, it can also lessen the likelihood that the radical willreact with other reaction components.

The invention is generally applicable to any of a variety of opticalassays that involve substantial illumination and/or photoactivatedconversion or excitation of chemical groups, e.g., fluorophores, and isparticularly useful for assays that are impaired by the generationand/or accumulation of triplet-state forms or free radicals. Forexample, the compositions and methods provided herein may be used withfluorescence microscopy, optical traps and tweezers, spectrophotometry,fluorescence correlation spectroscopy, confocal microscopy, near-fieldoptical methods, fluorescence resonance energy transfer (FRET),structured illumination microscopy, total internal reflectionfluorescence microscopy (TIRF), etc., all of which are well knowntechniques that are routinely used by those of skill in the art.

Further, the methods provided herein are particularly useful in analysesthat utilize very limited concentrations of reactants that might besubject to photo-induced damage, such as single moleculedetection/monitoring assays. As will be appreciated, in suchreagent-limited analyses, any degradation of a critical reagent willdramatically impact the analysis by further limiting the reagent, whichnot only can adversely effect the detectable signal, but may alsodirectly impact the reaction being monitored, e.g., by changing itsrate, duration, or product(s). For example, photo-induced damage caninclude a photo-induced change in a given reagent that reduces thereactivity of that reagent in the reaction, e.g., photobleaching of afluorescent molecule under excitation illumination, which diminishes orremoves its ability to act as a signaling molecule. Also included in theterm photo-induced damage are other changes that reduce a reactant'susefulness in a reaction, e.g., by making the reagent less specific inits activity in the reaction. Likewise, photo-induced damage includesundesired changes in a reagent that are caused by interaction of thatreagent with a product of another photo-induced reaction, e.g., thegeneration of singlet oxygen during a fluorescence excitation event,which singlet oxygen may damage organic or other reagents, e.g.,proteins. Such interaction may be a covalent or noncovalent interaction,a binding interaction, a transient interaction, a catalytic interaction,and the like. For example, damage to an enzyme that catalyzed a reactionbeing monitored may cause a reduction in the rate of the reaction, insome cases stopping it altogether, or may reduce the duration orfidelity of the reaction. As such, it is to be understood that referenceto photo-induced damage includes both direct damage to a reactant causedby optical energy (e.g., excitation radiation), as well as indirectdamage caused by interaction with another reactant that has undergonedirect or indirect photo-induced damage. It is to be understood thatexcitation radiation as used herein may comprise optical energy in theUV, visible, and/or IR range.

One particularly apt example of analyses that benefit from the inventionare single-molecule biological analyses, including, inter alia, singlemolecule nucleic acid sequencing analyses, single molecule enzymeanalyses, hybridization or binding assays (e.g., antibody assays),nucleic acid hybridization assays, nucleic acid sequencing assays,protein sequencing assays, polymerization assays, ligation reactions,catalytic reactions, and the like, where the reagents of primary importare subjected to prolonged illumination with relatively concentratedlight sources (e.g., lasers and other concentrated light sources, suchas mercury, xenon, halogen, or other lamps) in an environment whereexcitation/photoconversion is occurring with its associated generationof products. In certain embodiments, the methods, compositions, andsystems are used in nucleic acid sequencing processes that rely ondetection of fluorescent or fluorogenic reagents. Examples of suchnucleic acid sequencing technologies include, for example, SMRT™ nucleicacid sequencing (described in, e.g., U.S. Pat. Nos. 6,399,335,6,056,661, 7,052,847, 7,033,764, 7,056,676, 7,361,466, 7,416,844; U.S.Patent Publication No. 20100311061; and in Eid, et al. (2009) Science323:133-138, the full disclosures of which are incorporated herein byreference in their entireties for all purposes), non-real time, or “onebase at a time” sequencing methods available from, e.g., Illumina, Inc.(San Diego, Calif.), Helicos BioSciences (Cambridge, Mass.), ClonalSingle Molecule Array™, and SOLiD™ sequencing. Methods for singlemolecule protein sequencing are provided, e.g., in U.S. PatentPublication No. 20100317116, which is incorporated herein by referencein its entirety for all purposes. Such prolonged illumination can resultin photo-induced damage to reaction component and can diminish theireffectiveness in the desired reaction. Adding compounds of theinvention, e.g., a compound containing both a reducing and an oxidizingcenter, to the reaction mixtures can reduce the effects of thephoto-induced damage, e.g., by increasing the photo-induced damagethreshold period or in practical terms, increasing the read length of anucleic acid sequencing reaction.

II. Illuminated Analyses

Certain aspects of the invention are generally directed to mitigatingphoto-induced damage during the performance of illuminated analyses. Theterms “illuminated analysis” and “illuminated reaction” are usedinterchangeably and generally refer to an analytical reaction that isoccurring while being illuminated (e.g., with excitation radiation), soas to evaluate the production, consumption and/or conversion ofluminescent (e.g., fluorescent or fluorogenic) reactants and/orproducts. As used herein, the terms “reactant” and “reagent” are usedinterchangeably. In certain preferred embodiments, the illuminatedreaction is a sequencing reaction and the photo-induced damage results,directly or indirectly, from an excitation radiation source used todetect fluorescently labeled nucleoside polyphosphates as they are usedto extend to a nascent nucleic acid strand. In certain preferredembodiments, the illuminated reaction is a polypeptide sequencingreaction and the photo-induced damage results, directly or indirectly,from an excitation radiation source used to detect fluorescently labeledamino acids during nascent polypeptide strand synthesis. In certainpreferred embodiments, the illuminated reaction is a binding assay todetect association between an antibody and an antigen and thephoto-induced damage results, directly or indirectly, from an excitationradiation source used to detect a binding event, e.g., by observation ofone or more labels linked to the antibody and/or antigen. In certainpreferred embodiments, the illuminated reaction is a hybridization assayto detect association between two nucleic acids that share complete orpartial complementarity, and the photo-induced damage results, directlyor indirectly, from an excitation radiation source used to detect ahybridization event, e.g., by observation of one or more labels linkedto one or both of the nucleic acids.

The amount of time an illuminated analysis may be carried out beforephoto-induced damage so substantially impacts the reactants to renderthe analysis non-useful (e.g., when the reaction prematurely terminates)is referred to as the “photo-induced damage threshold period.” Aphoto-induced damage threshold period is assay-dependent, and isaffected by various factors, including but not limited tocharacteristics of reactants (e.g., enzymes, substrates, bindingpartners, etc.) in the assay (e.g., susceptibility to photo-induceddamage and the effect of such damage on enzyme activity/processivity),characteristics of the radiation source (e.g., wavelength, intensity),characteristics of the signal-generating molecule (e.g., type ofemission, susceptibility to photo-induced damage, propensity to entertriplet state, and the effect of such damage on the brightness/durationof the signal), and similar characteristics of other components of theassay. It can also depend on various components of the assay system,e.g., signal transmission and detection, data collection and analysisprocedures, etc. It is well within the abilities of the ordinarypractitioner to determine an acceptable photo-induced damage thresholdperiod for a given assay, e.g., by monitoring the signal decay for theassay in the presence of a photodamaging agent and identifying a periodfor which the signal is a reliable measure for the assay, and suchanalyses can optionally include time course reactions, titrations, andthe like. In certain preferred embodiments of the invention, thephoto-induced damage threshold period is that period of illuminatedanalysis during which such photo-induced damage occurs so as to reducethe rate, processivity, fidelity, product formation, or error frequencyof the subject reaction by no more than 10%, 20%, 30%, 40%, 50%, 60%,70%, 80%, or 90% over the same reaction in the absence of suchillumination. This impact on the subject reaction is typically due todirect or indirect damage to one or more reaction components, and ofparticular interest are those present in limiting quantities, e.g., at alow concentration. For example, certain single molecule reactionscomprise immobilized reactants that are present as a single molecule ata given reaction site. While other reactants in solution can diffuse inand out of the reaction site, such immobilized reactants are not“exchangeable” in the reaction mixture. As such, damage to theseimmobilized reactants is typically detrimental to the subject reactionat a given reaction site, and can even cause premature termination ofthe single reaction being monitored at a reaction site. It is an objectof the invention to increase the photo-induced damage threshold period,thereby increasing the amount of time reactions can proceed towardcompletion with minimal damage to the reactants, thereby lengthening thetime in which a detectable signal is an accurate measure of reactionprogression. In particular, it is an object to reduce damage toreactants at limiting concentrations, e.g., immobilized reactants, andespecially those present as single molecules at a reaction site.

In some contexts, a reaction comprising one or more components that havebeen subject to photo-induced damage may be subject to spuriousactivity, and thus be more active than desired. In such cases, it willbe appreciated that the photo-induced damage threshold period ofinterest would be characterized by that period of illuminated analysisduring which such spurious activity, e.g., as measured by an increase inreaction rate, or an increase in non-specific reaction rate, is no morethan 10% over a non-illuminated reaction, no more than 20% over anon-illuminated reaction, no more than 50% over a non-illuminatedreaction, and in some cases, no more than 90% over a non-illuminatedreaction. In one non-limiting example, where a nucleic acid polymerase,by virtue of a photodamaging event, begins to incorrectly incorporatenucleotides (or analogs or derivatives thereof) during template directedsynthesis, such activity would impact the photo-induced damage thresholdperiod as set forth above. In this case, the compounds and methods ofthe invention would increase the photo-induced damage threshold period,thus increasing the amount of time the reaction could be illuminatedbefore the above-described spurious activity occurred.

With reference to nucleic acid analyses, it has been observed that intemplate-directed synthesis of nucleic acids using fluorescentnucleotide analogs as a substrate, prolonged illumination can result ina substantial degradation in the ability of the polymerase to synthesizethe nascent strand of DNA, as described previously, e.g., in U.S.Published Patent Application No. 20070161017, incorporated by referenceherein in its entirety for all purposes. Damage to polymerase enzymes,template sequences, and/or primer sequences can significantly hinder theability of the polymerase to process longer strands of nucleic acids.For example, reduction in the processivity of a polymerase leads to areduction in read lengths for sequencing processes that identifysequence constituents based upon their incorporation into the nascentstrand. As is appreciated in the art of genetic analysis, the length ofcontiguous reads of sequence directly impacts the ability to assemblegenomic information from segments of genomic DNA. Such a reduction inthe activity of an enzyme can have significant effects on many differentkinds of reactions in addition to sequencing reactions, such asligations, cleavages, digestions, phosphorylations, other types ofpolymerizations, etc.

Without being bound to a particular theory or mechanism of operation, itis believed that at least one cause of photo-induced damage to enzymeactivity, particularly in the presence of fluorescent reagents, resultsfrom the direct interaction of the enzyme with photodamaged fluorescentreagents. Such interaction may be a covalent or noncovalent interaction,a binding interaction, a transient interaction, a catalytic interaction,and the like. Further, it is believed that this photo-induced damage ofthe fluorescent reagents (and possibly additional damage to the enzyme)is at least partially mediated by reactive intermediates (e.g., reactiveoxygen species) that are generated during the relaxation oftriplet-state fluorophores. One or both of the photo-induced damagedfluorescent reagents and/or reactive intermediates may be included inthe overall detrimental effects of photo-induced damage. One possiblemechanism for this photo-induced damage is shown in FIG. 1. As shown, afluorophore excited by exposure to electromagnetic radiation at anexcitation wavelength can transition into a triplet state. This mayoccur directly, or as a result of multi-photon processes, where anexcited fluorophore transitions to the triplet state upon contact with aphoton of a wavelength that is shorter (or bluer) than the nominalexcitation wavelength of the fluorophore. Subsequent relaxation of thetriplet-state fluorophore can lead to generation of reactiveintermediates, which can, in turn, damage one or both of the fluorophoreor the enzyme processing the fluorophore, e.g., the polymerase.Accordingly, photo-induced damage mitigating agents (e.g., free radicaland/or triplet-state quenching agents) are useful to prevent or slow theformation of reactive intermediates. Such agents can be included withinthe reaction mixtures or directly incorporated into compounds of theinvention, other reaction components, or confined at a reaction site toalleviate, prevent, and/or reverse the effects of reactiveintermediates, as well as other species generated during illuminatedreaction that can cause photo-induced damage, e.g., in single moleculereactions.

The photo-induced damage sought to be prevented by the methods andcompositions of the invention is not merely photo-induced damage tofluorescent reagents, e.g., photobleaching, but is also directed toprevention or reduction of the downstream effects of photoactivation ofsuch fluorescent reagents, e.g., during interaction with or proximity toother reagents, and in particular those that are of limited quantity ina reaction mixture, and as such, their limited presence is more greatlyimpacted by even slight losses due to photo-induced damage. For example,and without being bound to a theory of operation, photo-induced damageto reactive proteins, enzymes, or other reactants may include damage tothe reactants or irreversible interactions between such reactants andthe photo-induced damaged reagents. Such interactions may be covalent ornoncovalent interactions, binding interactions, transient interactions,catalytic interactions, and the like. As suggested by the foregoing,photo-induced damage generally refers to an alteration in a givenreagent, reactant, or the like, that causes such reagent to have alteredfunctionality in a desired reaction, e.g., reduced activity, reducedspecificity, or a reduced ability to be acted upon, converted, ormodified, by another molecule, that results from, either directly orindirectly, a photo-induced reaction, e.g., a photo-induced reactioncreates a reactant that interacts with and causes damage to one or moreother reactants. Typically, such a photoreaction directly impacts eitherthe reactant of interest, e.g., direct photo-induced damage, or impactsa reactant within one, two or three reactive steps of such reactant ofinterest. Further, such photoreaction can directly impact the reactionof interest, e.g., causing a change in rate, duration, processivity,product formation, or fidelity of the reaction.

In another aspect of the invention, the photo-induced damage mitigatingagents described herein are particularly suitable for mitigatingphoto-induced damage to reactants in small reaction volumeconcentrations, wherein such reactants may be present in solution, butat very limited concentrations, As generally referred to herein, suchlimited quantity reagents or reactants may be present in solution, butat very limited concentrations, e.g., less than 200 nM, in some casesless than 10 nM and in still other cases, less than 10 pM. In preferredaspects, however, such limited quantity reagents or reactants refer toreactants that are immobilized, or otherwise confined within a givenarea (a reaction site, e.g., within a confinement, e.g., a well,channel, or zero mode waveguide), so as to provide limited quantity ofreagents in that given area, and in certain cases, provide small numbersof molecules of such reagents within that given area, e.g., from 1 to1000 individual molecules, preferably between 1 and 10 molecules. Aswill be appreciated, photo-induced damage of immobilized reactants in agiven area will have a substantial impact on the reactivity of thatarea, as other, non-damaged reactants are not free to diffuse into andmask the effects of such damage.

While methods and compositions for limiting photo-induced damage tofluorophores have been previously provided, the negative impacts ofdownstream photodamage, e.g., to enzymes in the presence of or resultingfrom photodestruction of fluorescent reagents is a particular object ofthe present invention. The detrimental effect of the of a photodamageevent, whether from actual damage to a given reagent from thefluorophore or from interaction with a downstream damaged reagent isgenerally referred to herein as photodamage or photo-induced damage. Anyreagent that can prevent the photodamage is referred to herein as a“photoprotective reagent” or a “photo-induced damage mitigating agent.”

The present invention therefore provides, e.g., for use on suchsubstrates, photoprotective agents that reduce the level ofphoto-induced damage to these limited quantities of reagents. Inpreferred embodiments, such photoprotective agents comprise both areducing unit (e.g., a functional group comprising a reducing agent,center or moiety) and an oxidizing unit (e.g., a functional groupcomprising an oxidizing agent, center or moiety). Examplephotoprotective agents of the invention include, but are not limited tonitrobenzene derivatives or nitrobenzoic acid derivatives, e.g.,nitrobenzoic acid derivatives further comprising a thiol, a disulfidegroup, an aliphatic amine, or an aromatic amine.

III. Prevention or Mitigation of Photo-Induced Damage

In a first aspect, the invention is directed to methods and compositionsthat reduce, prevent, or reverse the amount of photo-induced damage toone or more reactants during an illuminated reaction, e.g., duringexcitation, the excited-state lifetime, or emission. In particular,compositions are provided that yield a reduction in the level ofphoto-induced damage (or an increase in the photo-induced damagethreshold period) as compared to such reactions in the absence of suchcompositions. As used herein, the components of such compositions thatprovide such effects are generally referred to as photo-induced damagemitigating agents or photoprotective agents. In particular,photo-induced damage mitigating agents are provided in the context ofthe illuminated reaction to reduce the level of photo-induced damage(and/or increase the photo-induced damage threshold period), that wouldotherwise have occurred but for the presence of the photo-induced damagemitigating agent.

Again, the definition of an agent as a photo-induced damage mitigatingagent is generally reflective of the impact that such agent has on theactual photo-induced damage event or the downstream impacts of thatdamage. As such, the detrimental impact of the photo-induced damageevent, whether resulting from actual damage to a given reagent or frominteraction with a damaged reagent, is generally referred to herein asphoto-induced damage. Such interaction may be a covalent or noncovalentinteraction, a binding interaction, a transient interaction, a catalyticinteraction, and the like. Therefore, a photo-induced damage mitigatingagent may prevent photo-induced damage of one or more reagents, or itmay mitigate the impact that a photo-induced damaged reagent may have ona particular, limited reagent in the reaction of interest. By way ofexample, an agent that blocks a detrimental interaction between aphoto-induced damaged fluorescent compound and a critical enzymecomponent (e.g., by quenching the triplet state of the fluorescentcompound) would still be referred to as a photo-induced damagemitigating agent, regardless of the fact that it did not prevent butrather reverted the initial photo-induced damage (triplet stateformation) to the fluorescent reagent.

Measurements of reduction of photo-induced damage as a result ofinclusion or treatment with one or more photo-induced damage mitigatingagents may be characterized as providing a reduction in the level ofphoto-induced damage over an untreated reaction. Further,characterization of a reduction in photo-induced damage generallyutilizes a measurement of reaction rates, durations, processivities,product formation, or fidelities, e.g., of enzyme activity, and/or acomparison of the photo-induced damage threshold period, between atreated reaction mixture and an untreated reaction mixture. Theseanalyses generally involve well established laboratory methods, such astime course reactions, titrations, and the like.

In the case of the present invention, the inclusion of photo-induceddamage mitigating agent(s) of the invention generally results in areduction of photo-induced damage of one or more reactants in a givenreaction, as measured in terms of “prevented loss of reactivity” in thesystem. Using methods known in the art, the amount of prevented loss ofactivity can be at least 10%, preferably greater than 20%, 30%, or 40%,and more preferably at least 50% reduction in loss of reactivity, and inmany cases greater than a 90% and up to and greater than 99% reductionin loss of reactivity. By way of illustration, and purely for thepurpose of example, when referring to reduction in photo-induced damageas a measure of enzyme activity in the presence and absence of thephoto-induced damage mitigating agent, if a reaction included a reactionmixture having 100 units of enzyme activity that would, in the absenceof a photo-induced damage mitigating agent and following illuminatedanalysis, yield a reaction mixture having only 50 units of activity,then a 10% reduction in photo-induced damage would yield a finalreaction mixture of 55 units (e.g., 10% of the 50 units otherwise lost,would no longer be lost). Similarly, “prevented loss of reactivity” canbe computed in terms of reaction rates, product formation, processivity,fidelity, and other metrics of a given analytical reaction.

IV. Photo-Induced Damage Mitigating Agents

Accordingly, in at least one aspect, the present invention is directedto an illuminated reaction volume that includes one or morephoto-induced damage mitigating agents that function to block orotherwise minimize the pathways that lead to such photo-induced damage,e.g., due to the creation of triplet-state fluorophores and resultingreactive oxygen species that can form during an illuminated reaction.Such agents that prevent photo-induced damage include reducing and/oroxidizing agents or anti-fade agents that reduce the lifetime and/orformation of the triplet-state fluorophores (also referred to astriplet-state quenchers), in some cases by interacting/reacting with atriplet-state fluorophore, thereby preventing its interaction with (andresulting photo-induced damage to) other reaction components. Suchagents also include oxygen and/or radical scavenging/quenching agentsthat remove oxygen, reactive oxygen species, and other radicals from thereaction mixture, thus preventing downstream damage to enzymes and/orother reaction components within the system. Such agents also includemixtures of agents having one or more reducing, oxidizing, anti-fade,triplet-state quenching, oxygen radical scavenging/quenching, or radicalscavenging/quenching activities. For example, in some preferredembodiments enzymatic systems for oxygen removal (oxygen scavenging) areused (e.g., protocatechuic acid/protocatechuate dioxygenase), optionallyin combination with other types of photo-induced damage mitigatingagents. Certain examples of photo-induced damage mitigating agents areprovided, e.g., in U.S. Published Patent Application Nos. 20070161017and 20100136592, the disclosures of which are incorporated herein byreference in their entireties for all purposes.

As described above, the drawbacks to using multicomponent ROXS includethe diffusion-limited, collision-based functionalities of these systems,which necessitate the use of relatively high concentrations of eachphotoprotective reagent (e.g., TSQ) to be effective, as well as theadverse effects of such high concentrations on certain analyticalreactions. The present invention provides a solution to the requirementfor high concentrations of ROXS components, and the detrimental effectsthat accompany them by combining the reducing agent and the oxidizingagent into a single photoprotective compound, termed a “single moleculeROXS.” This single molecule ROXS allows the reducing and oxidizingfunctions to occur in a single bimolecular interaction between thephotoprotective agent and the triplet state molecule so that theoxidizing and reducing reactions occur nearly simultaneously. As aresult, the single molecule ROXS can be used at a much lowerconcentration as compared to the multiple photoprotective reagents inthe conventional ROXS described above. For example, the concentration ofthe single molecule ROXS can be similar to that of a single member of aconventional ROXS pair. In certain embodiments, the single molecule ROXSis provided at a concentration of, e.g., 12 mM or lower, 10 mM or lower,8 mM or lower, 6 mM or lower, 4 mM or lower, 2 mM or lower, or 1 mM orlower.

Preferred embodiments of photoprotective agents of the invention arethose in which a reducing unit and an oxidizing unit are covalentlybound, or physically linked together to form a single molecule, e.g., awater-soluble molecule that can be added to the reaction mixture, e.g.,an illuminated reaction mixture. Particularly preferred examples areprovided in FIG. 2, and details on preparation of certain of thesecompounds are found herein in the examples section. In certain aspects,such a molecule comprises: (1) a reducing unit, such as a dialkylsubstituted analine, a thiol, an aliphatic amine, or an aromatic amine;and (2) an oxidizing unit, such as a substituted nitrobenzenederivative. Where a higher solubility of the compound is desired, thecompound can further comprise a water solubilizing unit, such as acarboxylic acid or sulfonyl, or a salt thereof (e.g., potassiumcarboxylate), or other agent that increases water solubility of thecompound (e.g., polyethylene glycol, etc.). For example, in someapplications a photoprotective agent may be insoluble or water solubleonly at low concentrations, so a water solubilizing unit can beincorporated to increase the solubility of the compound and allow higherconcentrations, e.g., in reaction mixtures and/or stock solutions. Incertain preferred embodiments, single molecule ROXS compounds compriseone or more rings, which can be homo- or heterocyclic rings, and whichare preferably five- or six-membered rings, but can comprise ahigher-order structure. In some embodiments, one or more of these unitsare linked to such rings, and they can be linked to the same ordifferent rings. Where both the oxidizing unit and reducing unit aredirectly or indirectly coupled to a single ring, a meta or paraorientation is preferable over an ortho orientation. For example, in anitrobenzene derivative compound of the invention, the nitrite group ispreferably linked to a carbon of the benzene ring that is at least twocarbons away from a carbon to which an amine, thiol, or disulfide groupis directly or indirectly linked. Where different units are linked todifferent rings, such rings are typically covalently attached togethervia a linker, examples of which are provided elsewhere herein. Incertain embodiments, the reducing unit is bound to a first ring and theoxidizing unit is bound to a second ring, where the first and secondrings are connected via a linker, e.g., a substituted or unsubstitutedalkyl chain. For example, a nitrobenzene derivative or nitrobenzoic acidderivative comprising a reducing unit, such as a thiol, disulfide group,or aliphatic or aromatic (homo- or heterocyclic) amine, can be used as asingle molecule ROXS compound. Of particular interest are compounds thatinclude a nitrobenzene derivative or nitrobenzoic acid derivative, e.g.,nitrobenzoic acid further comprising a thiol, disulfide, or amine groupto function as the reducing unit. The nitrite group and linkage to thereducing unit can be present at multiple different positions on thebenzene ring, and multiple nitrite groups can also be present, as shownin FIG. 2. Further, although certain of the compounds in FIG. 2 possessa carboxylic acid group as the water solubilizing unit, in certainpreferred embodiments a potassium carboxylate group or other carboxylatesalt is used, which can further increase the solubility of the compound.Likewise, although certain of the compounds in FIG. 2 possess apotassium carboxylate group as the water solubilizing unit, in certainembodiments a carboxylic acid group or other carboxylate salt is used inplace of the potassium carboxylate group. Other compounds can besimilarly constructed, e.g., by combining both methylviologen andascorbic acid into a single compound, e.g., using a linker, e.g., asdescribed elsewhere herein.

The terms oxidation and reduction describe chemical reactions in whichatoms have their oxidation number (oxidation state) changed, e.g., anatom can undergo reduction of oxidation number (reduction) or anincrease in oxidation number (oxidation). Substances that have theability to oxidize other substances are said to be oxidative and areknown as oxidizing agents, oxidants, or oxidizers. For example, anoxidant removes electrons from another substance, and is itself reduced.And, because it “accepts” electrons, it is also called an electronacceptor. Oxidizing agents in organic chemistry are those that increasethe oxygen content or decrease the hydrogen content of an organicmolecule. Substances that have the ability to reduce other substancesare said to be reductive and are known as reducing agents, reductants,or reducers. For example, a reducing agent transfers electrons toanother substance, and is itself oxidized. And, because it “donates”electrons, it is also called an electron donor. Electron donors can alsoform charge transfer complexes with electron acceptors. Reducing agentsare those that cause another compound to increase hydrogen content ordecrease its oxygen content. As used herein, a “reducing unit” includesor is a reducing agent, and an “oxidizing unit” is or includes anoxidizing agent.

Both types of groups are well known to those of skill in the art.Preferred reducing units that can be combined with oxidizing units tocreate photoprotective agents that are single compound TSQ reagents ofthe invention, include, but are not limited to, anilines (e.g.,N,N-dialkyl anilines, such as 4-(dimethylamino)phenylacetic acid or4-(dimethylamino)benzoic acid;3-(N-carboxyethylamino)-4-methoxy-nitrobenzoic acid; or3-(N,N-bis-carboxyethylamino)-4-methoxy-nitrobenzoic acid), ascorbicacid, anthracenes, thiols, disulfides, aromatic heterocyclic amines,aromatic homocyclic amines, aliphatic amines, dimethyl methylphosphonate(DMMP), dimethylaminopropylamine (DMAPA), and the like and derivativesand/or salts (e.g., potassium salts) thereof (e.g.,3-(N-carboxyethylamino)-4-methoxy-nitrobenzene, potassium salt; or3-(N,N-bis-carboxyethylamino)-4-methoxy-nitrobenzene, dipotassium salt).Other reducing agents that can be incorporated into a single moleculeROXS compound of the invention include, but are not limited to, N-propylgallate, mercaptoethanol,6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid and derivativesthereof, active metals such as potassium, calcium, barium, sodium andmagnesium, and compounds that contain a hydrogen ion, such as NaH, LiH₂LiAlH₄ and CaH₄, formic acid, oxalic acid, sulfites, and many othersknown to those of skill in the art.

Preferred oxidizing units that can be combined with reducing units tocreate photoprotective agents that are single compound TSQ reagents,include, but are not limited to, aromatic nitro derivatives such asnitrobenzoic acid, quinones, methylviologen, and the like andderivatives thereof. For example, in preferred embodiments a genericstructure of a compound of the invention is a nitro-substituted aniline,e.g.,

where each R can be a hydrogen atom (H) or comprise an alkyl group,e.g., an unbranched alkyl group, a branched alkyl group, a substitutedalkyl group, or a combination thereof. In certain preferred embodiments,however, the photoprotective agent does not comprise a quinone,hydroquinone, or a derivative thereof. Additional oxidizing compoundsthat can be incorporated into a single molecule ROXS compound of theinvention include, but are not limited to, halogen containing compounds,peroxides, hypochlorite, permanganate and magnate compounds, and manyother known to those of skill in the art. In certain embodiments,multiple reducing units and/or oxidizing units can be linked toconstruct a single molecule ROXS compound. For example, methylviologencan be linked to two reducing units, e.g., thiol groups.

The term “linker” encompasses any moiety that is useful to connect oneor more molecules or compounds (including, e.g., reducing units,oxidizing units, and/or water solubilizing units), e.g., to each otherand/or to a reaction site. For example, a linker can connect aphotoprotective agent (e.g., a triplet-state quencher, free radicalquencher, or single molecule ROXS described herein) to a reaction siteor a reaction component (e.g., an enzyme or fluorescent reactioncomponent); a linker can attach a reporter molecule or “label” (e.g., afluorescent dye) to a reaction site or a reaction component (e.g., anenzyme, substrate, ligand, binding partner, etc.); and a linker cancovalently link a reducing agent to a oxidizing agent to form a singlemolecule ROXS. Methods for choosing, synthesizing, and attaching linkersto reactants and surfaces are well known to those of ordinary skill inthe art and further discussion and exemplary linker moieties areprovided, e.g., in U.S. Ser. No. 61/026,992 (filed Feb. 7, 2008), U.S.Ser. No. 12/367,411, (filed Feb. 6, 2009), and U.S. Published PatentApplication No. 2009/0233302, the disclosures of which are incorporatedherein by reference in their entireties for all purposes. For example,by keeping a photoprotective agent in close proximity to a fluorescentlabel, the efficiency of suppressing and/or reversing triplet stateformation may be enhanced. (See, e.g., U.S. Patent Publication No.2009/0325260, which is incorporated herein by reference in its entiretyfor all purposes.) In certain embodiments, a photoprotective agent isbound to a linker that connects a fluorescent label to a reactant, suchas an enzyme substrate. A specific example would be to place aphotoprotective agent in a linker that connects a nucleosidepolyphosphate to a fluorescent dye:

Although designated at “dNTP” in the above structure, the nucleotidepolyphosphate may comprise three or more phosphate groups, and it may bea deoxyribonucleoside polyphosphate or a ribonucleoside polyphosphate.More than one photoprotective agent can be incorporated onto afluorescent dye molecule:

where “PA” is indicative of a photoprotective agent that is preferably atriplet state quencher, but may also be another type of photoprotectiveagent. In certain preferred embodiments, n=1, 2, or 3. One specificexample is:

where the photoprotective agent can be selected from those providedherein. In certain preferred embodiments, photoprotective agentsincluded within a labeled nucleoside polyphosphate construct areselected from:

The sulfonyl groups both increase the bulkiness of the labelednucleoside polyphosphate and its hydrophilicity. It is beneficial tokeep the photoprotective agent apart from the dye core to mitigate anyquenching, e.g., due to the nitro group, while allowing a proximitysufficient to enhance effective prevention or reversal of triplet stateformation.

Linkers may also be branched to connect three or more components of areaction mixture, e.g., in to a tridentate, tetradentate, or higherorder structure. For example, a dye may be linked to one or morephoto-induced damage mitigating agents, enzymes, or other reactioncomponents. In some such embodiments, a tridendate or higher order(e.g., tetradentate, pentadentate, hexadentate, etc.)) structure may beformed connecting the photo-induced damage mitigating agent to two ormore different reaction components and/or to a reaction site. Forexample, the photo-induced damage mitigating agent can be incorporatedinto a linker connecting two other components of the reaction. Methodsof producing such compounds are provided, e.g., in U.S. Ser. No.61/026,992 (filed Feb. 7, 2008) and U.S. Ser. No. 12/367,411, (filedFeb. 6, 2009), both of which are incorporated herein by reference intheir entireties for all purposes. In a single molecule sequencingreaction, such a tridendate structure may include a photo-induced damagemitigating agent, a dye, and a nucleoside polyphosphate, for example.Such an embodiment may be beneficial to bring the photo-induced damagemitigating agent near the dye to facilitate rapid quenching of anytriplet state occurring in the dye molecule. In certain preferredembodiments, a luminescently labeled reaction component (e.g.,fluorescent nucleoside polyphosphate, enzyme, etc.) is linked to atleast one photo-induced damage mitigating agent provided herein.

In certain embodiments, the linker is a member selected from substitutedor unsubstituted alkyl, substituted or unsubstituted heteroalkyl,substituted or unsubstituted aryl, substituted or unsubstitutedheteroaryl, substituted or unsubstituted cycloalkyl, and substituted orunsubstituted heterocycloalkyl. In one example, the linker moiety isselected from straight- and branched carbon-chains, optionally includingat least one heteroatom (e.g., at least one functional group, such asether, thioether, amide, sulfonamide, carbonate, carbamate, urea andthiourea), and optionally including at least one aromatic,heteroaromatic or non-aromatic ring structure (e.g., cycloalkyl,phenyl). In certain embodiments, molecules that have trifunctionallinkage capability are used, including, but are not limited to, cynuricchloride, mealamine, diaminopropanoic acid, aspartic acid, cysteine,glutamic acid, pyroglutamic acid, S-acetylmercaptosuccinic anhydride,carbobenzoxylysine, histine, lysine, serine, homoserine, tyrosine,piperidinyl-1,1-amino carboxylic acid, diaminobenzoic acid, etc.

The linker as a whole may comprise a single covalent bond or a series ofstable bonds. Thus, a reporter molecule (e.g., a fluorescent dye) may bedirectly attached to a triplet-state or free radical quencher of theinvention (e.g., a nitrobenzene derivative or nitrobenzoic acidderivative comprising a thiol or amine). A linker that is a series ofstable covalent bonds can incorporate non-carbon atoms, such asnitrogen, oxygen, sulfur and phosphorous, as well as other atoms andcombinations of atoms, as is known in the art. If the linker is notdirectly attached to a reactant by a single covalent bond, theattachment may comprise a combination of stable chemical bonds,including for example, single, double, triple or aromatic carbon-carbonbonds, as well as carbon-nitrogen bonds, nitrogen-nitrogen bonds,carbon-oxygen bonds, sulfur-sulfur bonds, carbon-sulfur bonds,phosphorus-oxygen bonds, phosphorus-nitrogen bonds, andnitrogen-platinum bonds. In an exemplary embodiment, the dye isconjugated to a nucleoside triphosphate as an alkylated tetraphosphateanalog. In other embodiments, other polyphosphate derivative can beused, e.g., a polyphosphate with 3 to 7 phosphate groups, wherein thepolyphosphate can further comprise a linker between a phosphate subunitand a fluorescent reporting moiety. A particularly preferred example isa dye labeled hexaphosphate derivative.

In certain preferred embodiments, linkers are derived from moleculeswhich comprise at least two reactive functional groups (e.g., one oneach terminus), and these reactive functional groups can react withcomplementary reactive functional groups on the various reactioncomponents or used to immobilize one or more reaction components at thereaction site. “Reactive functional group,” as used herein refers togroups including, but not limited to, olefins, acetylenes, alcohols,phenols, ethers, oxides, halides, aldehydes, ketones, carboxylic acids,esters, amides, cyanates, isocyanates, thiocyanates, isothiocyanates,amines, hydrazines, hydrazones, hydrazides, diazo, diazonium, nitro,nitriles, mercaptans, sulfides, disulfides, sulfoxides, sulfones,sulfonic acids, sulfinic acids, acetals, ketals, anhydrides, sulfates,sulfenic acids isonitriles, amidines, imides, imidates, nitrones,hydroxylamines, oximes, hydroxamic acids thiohydroxamic acids, allenes,ortho esters, sulfites, enamines, ynamines, ureas, pseudoureas,semicarbazides, carbodiimides, carbamates, imines, azides, azocompounds, azoxy compounds, and nitroso compounds. Reactive functionalgroups also include those used to prepare bioconjugates, e.g.,N-hydroxysuccinimide esters, maleimides and the like. Methods to prepareeach of these functional groups are well known in the art and theirapplication or modification for a particular purpose is within theability of one of skill in the art (see, for example, Sandler and Karo,eds. ORGANIC FUNCTIONAL GROUP PREPARATIONS, Academic Press, San Diego,1989).

In certain embodiments, the compounds provided herein may be used incombination with one another and/or with a variety of reducing agents,anti-fade agents, free radical quenchers/scavengers, oxygen scavengers,singlet oxygen quenchers, and/or triplet-state quenchers (e.g.,6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), including, forexample, those provided in U.S. Patent Publication Nos. 20070161017 and20100136592, previously incorporated by reference, which also providemethods of mitigating the impact of photo-induced damage on the resultsof a given analytical operation that may be used with the compounds andmethods of the provided herein.

In certain embodiments, a photo-induced damage mitigating agent is amixture of at least two different photoprotective compounds or acombination of derivatives of the same type of compound, e.g., at leastabout 2, 3, 4, 5, 6, or 7 single molecule ROXS compounds, e.g., asprovided in FIG. 2, can be used together to provide mitigation ofphoto-induced damage. Alternatively, additional mitigation compoundssuch as 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid,mercaptoethanol, methylviologen, ascorbic acid, quinones,dithiothreitol, and the like and derivatives thereof can be mixed withthe single molecule ROXS compounds of the invention. Such a mixture cancomprise various ratios of any two of its constituent components, e.g.,about 60:1, 40:1, 20:1, 10:1, 8:1, 6:1, 4:1, 2:1, 3:2, 1.5:1 1:1, 1:1.5,2:3, 1:2, 1:4, 1:6, 1:8, 1:10, 1:20, 1:40, 1:60, etc. For example, theremay be substantially equivalent amounts of multiple or each compound inthe mixture or a single component, e.g., one or more ROXS compounds, orone of the compounds may be mixed in a significantly higherconcentration than at least one other. In some embodiments, a mixture ofat least two different ROXS compounds provided herein comprises variouspercentages of its constituent compounds.

Additional photoprotective compounds can also be added to a reactionmixture. For example, 8-hydroxyjulolidine or derivatives thereof can beadded to a reaction mixture as a triplet state quencher or thesemolecules can be coupled to the dye molecule or a reactant in anilluminated reaction. Examples of hydroxyjulolidine derivatives that canbe used as TSQ reagents are provided in FIG. 3.

These approaches are particularly useful in the optical interrogation ofreactions where components of the reaction that are susceptible tophoto-induced damage are spatially confined on an assay plate orsubstrate, either through the presence of structural confinements,optical confinements, and/or through immobilization of the components.Examples of such confined reagents include surface immobilized orlocalized reagents, e.g., surface immobilized or associated enzymes,receptors, antibodies, etc. that are interrogated upon the surface,e.g., through fluorescence scanning microscopy or scanning confocalmicroscopy, total internal reflectance microscopy or fluorometry,surface imaging, or the like.

In accordance with the present invention, photo-induced damagemitigating agents may generally be provided as a component of thereaction mixture, either through addition as an additive, either liquidor solid, and can be predisposed and/or immobilized within the regionwhere the reaction is taking place, or may be provided in aconfiguration that permits them to freely interact with the aqueoussystem components by including such agents within or linked tostructures (e.g., caging groups, tridentate structures, etc.) thatrender the agents suspended in aqueous systems and additionallyavailable to interact with relevant portions of the reaction mixture,e.g., dissolved oxygen species. By way of example, in cases where thereaction of interest is confined to a particular region or location, itmay be desirable to immobilize or otherwise localize the photo-induceddamage mitigating agents within or proximal to that region, e.g., uponthe surfaces of the substrates or reactions wells. Likewise, where aphoto-induced damage mitigating agent comprises cooperativelyfunctioning components, e.g., dual enzyme systems, it may again bedesirable to localize such components relative to each other, as well asto the reaction of interest.

As used herein, a substrate may comprise any of a variety of formats,from planar substrates, e.g., glass slides or planar surfaces within alarger structure, e.g., a multi-well plates such as 96-well, 384-well,and 1536-well plates, or regularly spaced micro- or nano-poroussubstrates (e.g., arrays of zero mode waveguides). Such substrates mayalso comprise more irregular porous materials, such as membranes,aerogels, fibrous mats, or the like, or they may comprise particulatesubstrates, e.g., beads, spheres, metal or semiconductor nanoparticles,optical fibers, or the like.

In addition to the foregoing, it will be appreciated that the otherreagents in a given reaction of interest, including those reagents forwhich photo-induced damage is being mitigated in accordance with theinvention, may be provided in any of a variety of differentconfigurations. For example, they may be provided free in solution, orcomplexed with other materials, e.g., other reagents and/or solidsupports. Likewise, such reagents may be provided coupled to beads,particles, nanocrystals or other nanoparticles, or they may be tetheredto larger solid supports, such as matrices or planar surfaces. Thesereagents may be further coupled or complexed together with otherreagents, or as separate reagent populations or even as individualmolecules, e.g., that are detectably resolvable from other moleculeswithin the reaction space. In addition, for purposes of discussionherein, whether a particular reagent is confined by virtue of structuralbarriers to its free movement or is chemically tethered or immobilizedto a surface of a substrate, it will be described as being “confined.”For example, in some preferred embodiments, one or more reagents in anassay system are confined within an optical confinement. Such an opticalconfinement may be an internal reflection confinement (IRC) or anexternal reflection confinement (ERC), a zero mode waveguide, or analternative optical structure, such as one comprising porous film withreflective index media or a confinement using index matching solids.More detailed descriptions of various types of optical confinements areprovided, e.g., in International Application Publication No.WO/2006/083751, U.S. Pat. No. 6,917,726, and U.S. Pat. No. 7,170,050,the full disclosures of which are incorporated herein by reference intheir entireties for all purposes.

V. Exemplary Applications

As noted above, the methods and compositions of the invention are usefulin a broad range of illuminated analytical reactions, and particularlythose using photoluminescent or fluorescent reactants, and particularlysuch reactions where the reagents that are susceptible to photo-induceddamage are present at relatively low levels. One exemplary applicationof the methods and compositions described herein is in single moleculeanalytical reactions, where the reaction of a single molecule (or verylimited number of molecules) is observed in the analysis, such asobservation of the action of a single enzyme, receptor, or antibodymolecule. In particular, when an analysis relies upon a small populationof reagent molecules, damage to any significant fraction of thatpopulation will have a substantial impact on the analysis beingperformed. For example, prolonged interrogation of a limited populationof reagents, e.g., fluorescent analogs and enzymes, can lead tophoto-induced damage of the various reagents to the point ofsubstantially impacting the activity or functionality of the enzyme. Inparticular, it has been shown that prolonged illumination of DNApolymerases involved in synthesis using fluorescent nucleotide analogsresults in a dramatic decrease in the enzyme's ability to synthesizeDNA, often measured as a reduction in read length. Without being boundto any theory of operation, it is believed that in some cases thephoto-induced damage event affects the catalytic region of the enzymethus affecting either the ability of the enzyme to remain complexed withthe template, or its ability to continue synthesis. The compositions andmethods of the present invention can prevent or mitigate that impact byproviding photo-induced damage mitigating agents in the reactionmixture.

In general, the photo-induced damage mitigating agents described hereinare present in the reaction mixture at levels sufficient to providebeneficial impact, e.g., reduced photo-induced damage and/or extensionof the photo-induced damage threshold period, but are not present atlevels that interfere substantially with the reaction of interest, e.g.,the sequencing reaction. For example, the preferred compounds of theinvention combine an oxidizing unit and a reducing unit onto a singlemolecule to decrease the amount of photo-induced damage mitigatingreactants are needed. In certain preferred embodiments, thephoto-induced damage mitigating agents are present at 0.5-10.0 mM, ormore preferably between about 0.5 mM and 5 mM, which represents thetotal concentration of a single or a combination of photo-induced damagemitigating agents presented herein. However, these concentrations aremerely exemplary and may be change depending on various factorsincluding, e.g., the particular photo-induced damage mitigating agentand/or mixture thereof, the type of reaction to which it is added,conditions under which such reaction is to be performed, and the like.Such adjustments are well within the abilities of the ordinarypractitioner.

In another aspect, the present invention is directed to illuminatedreactions for single molecule analysis, including sequencing of nucleicacids by observing incorporation of nucleotides or nucleotide analogsinto a nascent nucleic acid sequence during template-directedpolymerase-based synthesis. Such methods, generally referred to as“sequencing-by-incorporation,” often involve the observation of theaddition of nucleotides or nucleotide analogs in a template-dependentfashion in order to determine the sequence of the template strand. See,e.g., U.S. Pat. Nos. 6,780,591, 7,037,687, 7,344,865, 7,302,146; U.S.Patent Publication Nos. 20100075327 and 20070036511; U.S. patentapplication Ser. No. 12/767,673, filed Apr. 26, 2010; U.S. patentapplication Ser. No. 12/635,618, filed Dec. 10, 2009; and Eid, et al.(2009) Science 323:133-138, all of which are incorporated herein byreference in their entireties for all purposes. Processes for performingthis detection typically include the use of fluorescently labelednucleotide analogs within a confined observation region, e.g., within ananoscale well and/or tethered, either directly or indirectly to asurface. By using excitation illumination (i.e., illumination of anappropriate wavelength to excite the fluorescent label and induce adetectable signal), the fluorescently labeled bases can be detected asthey are incorporated into the nascent strand, thus identifying thenature of the incorporated base, and as a result, the complementary basein the template strand. It will be understood that many different kindsof reactions can also benefit through use of the methods, compositions,and systems provided herein, e.g., including those described in U.S.patent application Ser. Nos. 12/813,968 and 12/814,075, both of whichwere filed Jun. 11, 2010, and are incorporated herein by reference intheir entireties for all purposes.

One particularly preferred aspect of the invention is in conjunctionwith the sequencing by incorporation of nucleic acids within an opticalconfinement, such as a zero mode waveguide. Such reactions involveobservation of an extremely small reaction volume in which one or only afew polymerase enzymes and their fluorescent substrates may be present.Zero mode waveguides, and their use in sequencing applications aregenerally described in U.S. Pat. Nos. 6,917,726 and 7,033,764, andpreferred methods of sequencing by incorporation are generally describedin Published U.S. Patent Application No. 2003-0044781, the fulldisclosures of which are incorporated herein by reference in theirentireties for all purposes, and in particular for their teachingsregarding such sequencing applications and methods. Briefly, arrays ofzero mode waveguides (“ZMWs”), configured in accordance with the presentinvention may be employed as optical confinements for single moleculeanalytical reactions, e.g., for nucleic acid (e.g., DNA, RNA) sequencedetermination. In particular, as noted above, these ZMWs provideextremely small observation volumes at or near the transparent substratesurface, also termed the “base” of the ZMW. A nucleic acid synthesiscomplex, e.g., template sequence, polymerase, and primer, which isimmobilized at the base of the ZMW, may then be specifically observedduring synthesis to monitor incorporation of nucleotides or nucleotideanalogs in a template-dependent fashion, and thus provide the identityand sequences of nucleotides or nucleotide analogs in the templatestrand. This identification is typically accomplished by providingdetectable label groups, such as fluorescent labeling molecules, on thenucleotides or nucleotide analogs. In some instances, the labelednucleotides or nucleotide analogs terminate primer extension, allowing a“one base at a time” interrogation of the complex. If, upon exposure toa given labeled base, a base is incorporated, its representativefluorescent signal may be detected at the base of the ZMW. If no signalis detected, then the base was not incorporated and the complex isinterrogated with each of the other bases, in turn. Once a base isincorporated, the labeling group is removed, e.g., through the use of aphotocleavable linking group or enzymatic cleavage of the alphaphosphate, and where the label was not the terminating group, aterminator, upon the 3′ end of the incorporated nucleotide or nucleotideanalog, may be removed prior to subsequent interrogation.

In accordance with the present invention, the above-described sequencingreaction may be carried out in the presence of one or more photo-induceddamage mitigating agents (e.g., single molecule ROXS compounds andconjugates and mixtures thereof) provided herein, either alone or incombination with other reaction mixture additives, such as reducingagents, antifade agents, free radical quenchers, triplet-statequenchers, singlet oxygen quenchers, or enzyme systems for depletion ofoxygen species (e.g., comprising an oxidase). In certain preferredembodiments, the sequencing reactions may be carried out in the presenceof at least one of the photoprotective agents described herein. Forexample, a photo-induced damage mitigating agent may be a compound inwhich a reducing unit is covalently bound to an oxidizing unit to form asingle molecule ROXS triple state quencher.

In another aspect, the illuminated reaction mixture includes anucleoside polyphosphate connected to a fluorescent dye by a linker. Thelinker in such a reaction mixture itself may comprise one or morephoto-induced damage mitigating agents, such as single molecule ROXScompounds, hydroxyjulolidene derivatives, oxidizing units, reducingunits, or mixtures thereof.

In addition to the use of photo-induced damage mitigating agents, thepresent invention also provides alternative methods of mitigating theimpact of photo-induced damage on a reaction. Such alternative methodscan be used in combination with the compositions and methods describedabove to further alleviate the effects of species that can be generatedduring an illuminated reaction.

One alternative method of mitigating the impact of photo-induced damageon the results of a given reaction is by only interrogating a reactionmixture, e.g., detecting fluorescent emission, during such portion ofthe illumination period before which excessive photo-induced damage hasoccurred. This approach is particularly useful in the opticalinterrogation of reactions where components of the reaction that aresusceptible to photo-induced damage are spatially confined on an assayplate or substrate, either through the presence of structuralconfinements and/or through immobilization of the components. Examplesof such confined reagents include surface immobilized or localizedreagents, e.g., surface immobilized or associated enzymes, antibodies,etc. that are interrogated upon the surface, e.g., through fluorescencescanning microscopy or scanning confocal microscopy, total internalreflectance microscopy or fluorometry, surface imaging, or the like. Thetiming of observation of each reaction is determined based on knowledgeof the photodamage threshold period, as described elsewhere herein. Agiven reaction or set of reactions is monitored under illumination for aperiod that is less than the photodamage threshold period, followed by aredirection of the illumination to a second, preferably adjacent,reaction or set of reactions, which were not previously subjected to theillumination. This second reaction or set of reactions is also observedfor a period less than the photodamage threshold period, before theillumination is redirected to a third reaction or set of reactions, andso on. Further details are provided in U.S. Patent Publication No.20070161017, incorporated herein by reference in its entirety for allpurposes.

Another alternative method of mitigating the impact of photo-induceddamage on the results of a given reaction provides for the eliminationof potentially damaging oxygen species using means other than the use ofthe photo-induced damage mitigating agents described above. In oneexample, dissolved oxygen species may be flushed out of aqueous systemsby providing the reaction system under different gas environments, suchas by exposing an aqueous reaction to neutral inert gas environments,such as argon, nitrogen, helium, xenon, or the like, to preventdissolution of excess oxygen in the reaction mixture. By reducing theinitial oxygen load of the system, it has been observed thatphoto-induced damage effects, e.g., on polymerase mediated DNAsynthesis, is markedly reduced. In particularly preferred aspects, thesystem is exposed to a xenon atmosphere. In particular, since xenon canbe induced to form a dipole, it operates as a triplet-state quencher inaddition to supplanting oxygen in the aqueous system. (See, e.g.,Vierstra and Poff, Plant Physiol. 1981 May; 67(5): 996-998, which isincorporated herein by reference in its entirety for all purposes) Assuch, xenon would also be categorized as a quencher, as set forth above.

It is to be understood that the above description is intended to beillustrative and not restrictive. It readily should be apparent to oneskilled in the art that various embodiments and modifications may bemade to the invention disclosed in this application without departingfrom the scope and spirit of the invention. For example, in certainembodiments various photo-induced damage mitigating agents and systemscan be combined within a single reaction mixture, in particular wheretheir modes of action differ and/or complement one another. The scope ofthe invention should, therefore, be determined not with reference to theabove description, but should instead be determined with reference tothe appended claims, along with the full scope of equivalents to whichsuch claims are entitled. All publications mentioned herein are citedfor the purpose of describing and disclosing reagents, methodologies andconcepts that may be used in connection with the present invention.Nothing herein is to be construed as an admission that these referencesare prior art in relation to the inventions described herein. Throughoutthe disclosure various patents, patent applications and publications arereferenced. Unless otherwise indicated, each is incorporated byreference in its entirety for all purposes.

VI. Examples

The following non-limiting examples illustrate methods of making andusing various photoprotective compounds of the invention.

Preparation of

A mixture of 1.23 g of mono-methyl-5-nitroisophthalate, 2.41 g ofN,N,N′,N′-Tetramethyl-O—(N-succinimidyl)uronium tetrafluoroborate and1.4 mL of triethylamine was stirred in 25 mL of DMF at room temperaturefor 15 minutes. In a separate flask, an aqueous solution of 3.13 g oftaurine in 30 mL of 1 M NaHCO3 was prepared. The DMF solution was thenadded to the aqueous solution via a separatory funnel and the mixturewas stirred at room temperature for about an hour. All volatilecomponents were evaporated and the crude material was purified on asilica gel column eluting with acetonitrile and water to yield 1.09 g ofproduct.

Preparation of

To 0.675 g of (I)-65 in 15 mL of water, 0.91 mL of triethylamine wasadded and the mixture was stirred at room temperature for 4 hours.Volatile components were removed under reduced pressure to recover theproduct which can be used without further purification.

Preparation of

To 1.089 g of SY360-(II) in 20 mL of DMF at room temperature, 0.65 g ofN,N,N′,N′-Tetramethyl-O—(N-succinimidyl)uronium tetrafluoroborate and0.84 mL of triethylamine were added and stirred t room temperature for20 minutes. At the end of the period, 0.35 g of cyteamine was added andstirred for 1 hour. Volatile components were removed under reducedpressure and the crude was purified on a silica gel column eluting withacetonitrile and water to yield 0.173 g of the desired product.

Preparation of

A mixture of 0.2 g of 2-chloro-5-nitrobenzoic acid, 0.462 g ofdithiothreitol (DTT), and 0.35 mL of triethylamine in 2 mL of DMF washeated at 50 C for 20 hours. The reaction was then diluted with about 30mL of 1 N HCl and extracted with ethyl acetate and dried over magnesiumsulfate. The crude product thus obtained was purified on HPLC to obtain0.235 g of product as its triethylammonium salt which was replaced bypotassium by adding 1 equivalent of KOH in aqueous medium.

Preparation of

This compound was prepared by starting with a mixture of 0.2 g of2-bromo-3-nitrobenzoic acid, 0.462 g of dithiothreitol (DTT), and 0.35mL of triethylamine in 2 mL of DMF. The mixture is then heated at 50 Cfor 20 hours. The reaction is then diluted with about 30 mL of 1 N HCland extracted with ethyl acetate and dried over magnesium sulfate. Thecrude product thus obtained can be purified on HPLC to obtain theproduct as its triethylammonium salt which can be replaced by potassiumby adding 1 equivalent of KOH in aqueous medium.

Preparation of

This compound was prepared by starting with a mixture of4-chloro-3-nitrobenzoic acid, dithiothreitol (DTT), and triethylamine in2 mL of DMF. The mixture is then heated at 50 C for 20 hours. Thereaction is then diluted with about 30 mL of 1 N HCl and extracted withethyl acetate and dried over magnesium sulfate. The crude product thusobtained can be purified on HPLC to obtain the product as itstriethylammonium salt which can be replaced by potassium by adding 1equivalent of KOH in aqueous medium.

Preparation of

This compound can be prepared by starting with a mixture of3-chloro-2-nitrobenzoic acid, dithiothreitol (DTT), and triethylamine in2 mL of DMF. The mixture is then heated at 130 C for 20 hours. Thereaction is then diluted with about 30 mL of 1 N HCl and extracted withethyl acetate and dried over magnesium sulfate. The crude product thusobtained can be purified on HPLC to obtain the product as itstriethylammonium salt which can be replaced by potassium by adding 1equivalent of KOH in aqueous medium.

Preparation of N,N′-dimethyl-N,N′-bis(mercaptoacetyl)hydrazine (DMH)

The compound was prepared by following the literature procedure (R.Singh and G. Whitesides, J. Org. Chem., vol 56, 2332-2337, 1991).

Preparation of

This compound can be prepared by starting with a mixture of2-chloro-5-nitrobenzoic acid, DMH, and triethylamine in 2 mL of DMF. Themixture is then heated at 50 C for 20 hours. The reaction is thendiluted with about 30 mL of 1 N HCl and extracted with ethyl acetate anddried over magnesium sulfate. The crude product thus obtained can bepurified on HPLC to obtain the product as its triethylammonium saltwhich can be replaced by potassium by adding 1 equivalent of KOH inaqueous medium.

Preparation of

This compound can be prepared by starting with a mixture of2-chloro-5-nitrobenzoic acid, 4-(dimethylamino)thiophenol, andtriethylamine in 2 mL of DMF. The mixture is then heated at 130 C for 20hours. The reaction is then diluted with about 30 mL of 1 N HCl andextracted with ethyl acetate and dried over magnesium sulfate. The crudeproduct thus obtained can be purified on HPLC to obtain the product asits triethylammonium salt which can be replaced by potassium by adding 1equivalent of KOH in aqueous medium

Preparation of 1,1′-bis(6-deoxyascorbate)-4,4′-bipyridinium dibromide

The compound was prepared by reacting 4,4′-bipyridine with6-bromo-6-deoxy-ascorbic acid (J. Med. Chem., E. Schmid, V. Figala, D.Roth and V. Ullrich, vol 36, 4021-4029, 1993).

Sequencing Performance of Two-Component ROXS Vs. Single Molecule ROXSCompound

Experiments were conducted using a Single Molecule Real Time (SMRT™)four-color sequencing instrument. (For detailed information onexperiments, see, e.g., Eid, et al. (2009) Science 323:133-138.)Briefly, 30 nM of a phi29 polymerase enzyme modified for immobilizationwas mixed with 10 nM of a circular DNA template/primer complexes andother reaction mixture components, including Ca2+ salt and nucleotideanalogs bearing a phospholinked fluorescent dye in MOPS buffer, pH 7.5.The mixture was incubated at 37° C. to allow formation ofpolymerase/template/primer complexes. The mixture was then diluted andan aliquot was added to a zero mode waveguide array, which was incubatedto allow immobilization of the complexes within zero mode waveguides onthe array. After washing, a solution comprising fluorescently labelednucleotides was added and the array was placed inside the sequencinginstrument. Sequencing was initiated and twenty minute reactions weremonitored in real time. The fluorescence emissions were recorded,processed, and analyzed. FIG. 5 shows accuracy distributions for suchreactions in the presence of a two-component ROXS (panel A), a singlemolecule ROXS compound (panel B), and in the absence of both atwo-component ROXS and a single molecule ROXS compound (panel C). Thechemical structures for each of these compounds is provided to the leftof the distribution. Although the maxima of the accuracy distributionsin the presence of a two-component or single molecule ROXS are similar,the concentrations of each required to achieve these accuracydistributions were quite different. The two-component ROXS was composedof 6 mM potassium nitrobenzoate (NBA) and 2.5 mM DMAPA, which the singlemolecule ROXS compound was present at a concentration of only 0.5 mM.The maxima of accuracy distributions in the presence of either atwo-component or single molecule ROXS were significantly higher thanwhen neither was present.

Sequencing Performance of Two Different Single Molecule ROXS Compounds

Experiments were conducted using a Single Molecule Real Time (SMRT™)four-color sequencing instrument as described above, and the data shownare based on seven minute reactions monitored in real time. FIG. 6 showsaccuracy distributions for these reactions in the presence of a twodifferent single molecule ROXS compounds (panels A and B) and in theabsence of both (panel C), as well as the chemical structures of thecompounds. The single molecule ROXS compounds were present at aconcentration of 0.25 mM, which is notably even lower than theconcentration of the single molecule ROXS compound used in theexperiment described above. The maxima of the accuracy distributions inthe presence of these single molecule ROXS were similar to one another,and were significantly higher than when neither was present.

Sequencing Performance of Two Different Two-Component ROXS Vs. TwoDifferent Single Molecule ROXS Compounds

Experiments were conducted using a Single Molecule Real Time (SMRT™)four-color sequencing instrument as described above, and the data shownare based on seven minute reactions monitored in real time. FIG. 7 showsaccuracy distributions for these reactions in the presence of a twodifferent two-component ROXS (panels A and B), and two different singlemolecule ROXS compounds (panels C and D). Panel A shows the accuracydistribution in the presence of 6 mM NBA and 2 mM mercaptoethylamine(MEA); panel B shows the accuracy distribution in the presence of 6 mMNBA and 2.5 mM DMAPA; and panels C and D show the accuracy distributionsin the presence of 0.25 mM of two different single molecule ROXScompounds, whose chemical structures are provided to the left of thedistributions. Although the maxima of the accuracy distributions in thepresence of a two-component ROXS or single molecule ROXS compounds weresimilar, the concentrations of the single molecule ROXS compoundsrequired to achieve these accuracy distributions were much lower thanthe concentrations of the two-component ROXS.

Sequencing Performance of a Two-Component ROXS Vs. Six Different SingleMolecule ROXS Compounds

Experiments were conducted using a Single Molecule Real Time (SMRT™)four-color sequencing instrument as described above, and the data shownare based on fifteen minute reactions monitored in real time. FIG. 8shows accuracy distributions for these reactions in the presence of atwo-component ROXS, and six different single molecule ROXS compounds, aswell as the chemical structures of the compounds. Panels A and E showthe accuracy distribution in the presence of 6 mM NBA and 2.5 mM DMAPA;and panels B, C, D, F, G, and H show the accuracy distributions in thepresence of 0.25 mM of two different single molecule ROXS compounds,whose chemical structures are provided to the left of the distributions.Once again, although the maxima of the accuracy distributions in thepresence of a two-component ROXS or single molecule ROXS compounds weresimilar in these experiments, the concentrations of the single moleculeROXS compounds required to achieve these accuracy distributions weremuch lower than the concentrations of the two-component ROXS.

Sequencing Performance of Four Different Single Molecule ROXS Compounds

Experiments were conducted using a Single Molecule Real Time (SMRT™)four-color sequencing instrument as described above, and the data shownare based on seven minute reactions monitored in real time. FIG. 9 showsaccuracy distributions for these reactions in the presence of fourdifferent single molecule ROXS compounds (panels A-D), and the chemicalstructures of the compounds are provided to the left of thedistributions. The single molecule ROXS compounds are each present at aconcentration of 0.25 mM, which is significantly lower than the typicalconcentrations of either member of a two-component ROXS. This experimentdemonstrated that single molecule ROXS compounds having structuralvariations can perform similarly in improving the accuracy of sequencingreactions.

What is claimed is:
 1. A method for protecting an enzyme fromphoto-induced damage in an illuminated reaction, the method comprising:providing a reaction mixture comprising the enzyme, and a fluorescent orfluorogenic substrate for the enzyme, wherein interaction of the enzymeand the fluorescent or fluorogenic substrate under excitationillumination results in altered activity of the enzyme; adding aphotoprotective agent to the reaction mixture, wherein thephotoprotective agent comprises a compound of a formula selected fromthe group consisting of:

 and a carboxylate salt thereof; and illuminating the reaction mixturewith an excitation illumination, wherein the photoprotective agentreduces an amount of photo-induced damage to the enzyme resulting frominteraction of the enzyme with the fluorescent or fluorogenic substrateunder the excitation illumination to an amount that is less than thatwhich would occur in the absence of the photoprotective agent.
 2. Themethod of claim 1, further comprising the step of monitoring a reactionbetween the enzyme and the fluorescent or fluorogenic substrate whileilluminating the reaction mixture.
 3. The method of claim 1, whereinsaid illuminated reaction is a base extension reaction.
 4. The method ofclaim 1, wherein the enzyme is a polymerase or a ligase.
 5. The methodof claim 1, wherein the reaction mixture further comprises a templatenucleic acid molecule.
 6. The method of claim 1, wherein the fluorescentor fluorogenic substrate comprises a nucleoside polyphosphate or analogthereof.
 7. The method of claim 1, wherein at least one component of thereaction mixture is confined within a zero mode waveguide.
 8. The methodof claim 1, wherein illuminating the reaction mixture comprisesilluminating the reaction for a period of time that is less than aphoto-induced damage threshold period, wherein the photo-induced damagethreshold period is lengthened in the presence of the photoprotectiveagent.
 9. The method of claim 1, wherein the carboxylate salt is apotassium salt.
 10. A method for increasing the accuracy of anilluminated sequencing reaction, the method comprising: providing areaction mixture comprising a polymerase, a template nucleic acid, and afluorescent or fluorogenic nucleotide or nucleotide analog; adding aphotoprotective agent to the reaction mixture, wherein thephotoprotective agent comprises a compound of a formula selected fromthe group consisting of:

 and a carboxylate salt thereof; illuminating the reaction mixture withan excitation illumination; and detecting emission signals from thereaction mixture, wherein the photoprotective agent enhances thedetection, thereby increasing the accuracy of the sequencing reaction.11. The method of claim 10, further comprising the step of monitoring areaction between the polymerase and the fluorescent or fluorogenicnucleotide or nucleotide analog while illuminating the reaction mixture.12. The method of claim 10, wherein the photoprotective agent reduces anamount of blinking or photobleaching of a dye within the fluorescent orfluorogenic nucleotide or nucleotide analog resulting from excitation ofthe dye by the excitation illumination to an amount that is less thanthat which would occur absent the photoprotective agent.
 13. The methodof claim 10, wherein said illuminated sequencing reaction is a baseextension reaction.
 14. The method of claim 10, wherein at least onecomponent of the reaction mixture is confined within a zero modewaveguide.
 15. The method of claim 10, wherein the polymerase isconfined within a zero mode waveguide.
 16. The method of claim 10,wherein the carboxylate salt is a potassium salt.